1. Field of the Invention
This invention relates to semiconductor packaging technology, and more particularly, to a method and an apparatus for inspecting solder balls on a BGA (Ball Grid Array) package, which are capable of rejecting defectively-bonded solder balls while allowing properly-bonded ones to pass therethrough.
2. Description of Related Art
BGA (Ball Grid Array) is an advanced type of semiconductor packaging technology which is characterized in the use of a substrate whose front side is mounted with a semiconductor chip and whose back side is implanted with a grid array of solder balls. During SMT (Surface Mount Technology) processes, the BGA package can be mechanically bonded and electrically coupled to an external printed circuit board (PCB) by means of these solder balls.
Fundamentally, a solder ball is said to be properly bonded if the bonding strength between the solder ball and its solder-ball pad is adequate enough to withstand a certain magnitude of shear. For example, for a solder ball of 0.75 mm in diameter, it should be at least able to withstand a ball shear of 1 kg; otherwise, it would be regarded as defectively bonded. A defectively-bonded solder ball would be easily broken apart from its solder-ball pad during transportation and handling. By conventional BGA fabrication, however, solder balls may often be defectively bonded due to the solder-ball pads being oxidized or contaminated by chemicals during the fabrication processes. This problem is illustratively depicted in the following with reference to FIGS. 1A-1C.
FIG. 1A shows the structure of a typical BGA package 10. As shown, this BGA package 10 is constructed on a substrate 11 whose front side is mounted with a semiconductor chip 12 and whose back side is formed with an array of solder-ball pads 13 masked by a solder mask 14. The solder-ball pads 13 are typically plated with gold (Au) to increase the solder-wettability thereof.
During a ball-implantation process, a grid array of solder balls 15 are attached to the respective solder-bail pads 13 and then reflowed to allow them to be wetted to the same. This allows the solder balls 15 to be mechanically and electrically bonded to the respective solder-ball pads 13.
As illustrated in FIG. 1B, during earlier BGA fabrication processes, however, the solder-ball pads 13 may be degraded in solder-wettability due to the existence of some wettability-degrading substances 16 over the surface thereof, which would degrade the solder-wettability of the solder-ball pads 13. The wettability-degrading substances 16 include, for example, the oxidation of the Au-plated surface of the solder-ball pads 13 and the remnant mask material and etchant that are left over the solder-ball pads 13 after the solder mask 14 is fabricated through a selective etching process. Since these wettability-degrading substances 16 are electrically insulative and non-wettable to solder, they would considerably degrade the solder-wettability of to solder-ball pads 13.
As further illustrated in FIG. 1C, if one of the solder-ball pads 13 is degraded in solder-wettability to a certain degree, it would undesirably cause the subsequently implanted solder ball 15 thereon to be defectively bonded; and consequently, during transportation or handling, the defectively-bonded solder ball 15 can be easily broken apart from its solder-ball pad 13. Even if the defectively-bonded solder ball 15 remains in position during transportation and handling, the wettability-degrading substances 16 would nevertheless cause an unreliable electrical coupling between the solder ball 15 and the solder-ball pad 13, making the finished BGA package 10 unreliable to use.
There exists therefore a need for a method and apparatus for use in quality control of implanted solder balls on a BGA package and which is capable of rejecting defectively-bonded solder balls while allowing properly-bonded ones to pass therethrough.
It is therefore an objective of this invention to provide a method and an apparatus for inspecting solder balls on a BGA package, which are capable of rejecting defectively-bonded solder balls while allowing properly-bonded ones to pass therethrough.
In accordance With the foregoing and other objectives, the invention proposes a novel method and apparatus for inspecting solder balls on BGA package.
In terms of method, the invention comprises the following steps (1) providing a rotatable disk having at least one flap on the perimeter thereof, the rotatable disk being capable of being rotated if a tangential force applied to the flap is greater than a predefined threshold torque set to be equal to or less than the shear-resistant strength of a properly-bonded solder bail but greater than the shear-resistant strength of a defectively-bonded one; (2) providing a conveyance path for conveying the BGA package; and (3) positioning the rotatable disk above the conveyance path, with the flap thereon being suspended above the conveyance path to allow the solder ball on the BGA package to push tangentially against the flap; wherein the solder ball would be capable of pushing open the flap if it is a properly-bonded one and would be rejected and broken apart from the BGA package by the flap if it is a defectively-bonded one.
In terms of apparatus, the invention comprises: (a) a conveyance belt for conveying the BGA package; (b) a rotatable disk having at least one flap on the perimeter thereof; the rotatable disk being capable of being rotated if a tangential force applied to the flap, is greater than a predefined threshold torque set to be equal to or less than the shear-resistant strength of a properly-bonded solder ball but greater than the shear-resistant strength of a defectively-bonded one; and the rotatable disk being fixed above the conveyance belt with the flap being suspended above the conveyance path to allow the solder ball on the BGA package to push tangentially against the flap; wherein the solder ball would be capable of pushing open the flap if it is a properly-bonded one and would be rejected and broken apart from the BGA package by the flap if it is a defectively-bonded one.
By the method and apparatus of the invention, as a BGA package has undergone the inspection procedure through the rotatable disk, all the properly-bonded ones of the solder balls on the BGA package would remain in position over the BGA package, while all the defectively-bonded ones would be removed.